A capacitor (such as an electric double layer capacitor, a hybrid capacitor or a redox capacitor) has a characteristic that a terminal voltage largely fluctuates depending on a charged/discharged state thereof, as compared to conventional secondary batteries. Generally, an electronic device is configured such that an operating voltage thereof has a certain allowable range. However, if a supply voltage changes to a value out of the operating voltage range, the electronic device becomes operationally unstable or becomes non-operative. Therefore, in cases where a capacitor having a large voltage fluctuation is used as a power supply of an electronic device, it is necessary to keep the voltage fluctuation in the capacitor within a given range.
As means for keeping an output voltage constant, a technique is conceivable which is designed to perform voltage conversion using a DC-DC converter as illustrated in FIG. 1. However, when a DC-DC converter is operated in a wide voltage range, problems, such as a significant increase in loss, and an increase in circuit size, will occur. The problems also occur during charging to the capacitor. A charger is a sort of DC-DC converter, thus, charging is performed in a wide voltage range, loss in the charger will increase, and the size of the charger will become larger. Considering that a capacitor has an energy density which is significantly less than those of conventional secondary batteries, it is desirable to establish a voltage conversion scheme with higher efficiency as compared to a conventional DC-DC converter.
For this purpose, there has been proposed a capacitor-based power supply apparatus which comprises a capacitor module composed of a plurality of capacitors connected in series, and a plurality of taps, as illustrated in FIG. 2, wherein the apparatus is configured to change the number of ones of the series-connected capacitors which are to be connected to a load (to change the taps), to thereby reduce a fluctuation range of an output voltage thereof (the following Patent Document 1).    Patent Document 1: JP 2000-209775 A    Patent Document 2: JP 2008-219964 A    Non-Patent Document 1: K. Z. Guo, et al.: “Comparison and evaluation of charge equalization technique for series connected batteries” in Proc. IEEE PESC'06, pp. 1-6 (2006)